I'm new to Android TV development. I'm looking to build my own TV interface that will allow me to create my own home screen with the apps I choose from the App Store, my own settings screen, ect. I don't want the user to be able to get out of my custom app and the main OS. I basically want to create my own flavor of the OS. Is this possible to do as an app that runs at startup and takes over the device, or would I be looking at creating my own version of the OS? And is it possible to create my own version of the OS and use apps from the Google Play store? Thanks
I don't think this is possible. In Google's documentation you can find information about all the things that you can do and as you will see, you are not given such wide freedom as the one you are seeking.
You can however, change through your app the content of the home screen but not the layout. Meaning that you can, for example, add a new recommendation on the home screen as described here. But, this is as far as you can get.
Edit: If you just want to run your app in kiosk mode on an Android TV device, you can do so, the same way you do it on phones or tablets. For example, scalefusion supports android TVs, but you should google to find more options and compare them in detail.